The present invention relates to apparatus for and a method of producing a beam of ions for ion implantation in a target element. The invention relates in particularly but not exclusively to apparatus for and methods of implanting ions of a preselected species, into a target element, for example the controlled introduction of a species into the surface layer of another material. The technique is important in semi-conductor technology where it is used in the manufacture of integrated circuits and other devices, particularly by modifying the conductivity of semi-conductor material by introducing chemical impurities into the material. The general background to the use of ion implantation in the manufacture of large scale integrated circuit chips is set out in published U.S. Pat. No. 4578589, having the same inventor as the present application.
An ion implantation apparatus normally consists of an ion source, an extraction system having an extraction-electrode which accelerates the ions from the extraction slot in the ion source, an analysing magnet for the selection of the required ion species, acceleration stages before or after the analysing magnet, and a target region with means for producing relative scanning movement between the beam and the target, either by scanning the beam, or by moving the target.
A number of problems arise in connection with the ion source and extraction system, particularly where the ion implantation apparatus is to be used to implant a number of different species selectively in a target. The ion source is typically a hot cathode, arc discharge, with a tungsten filament, maintained at approximately 10.sup.-2 torr pressure by a gas or vapour supply system. Such an ion source has a limited operational life time before it requires servicing, and its performance and reliability are adversely affected by using the source for a number of different species. The extraction system is typically an accelerating gap with a voltage of 10-50 kV across it. Sparking across this gap often becomes a problem when the extraction conditions are changed, particularly when the species is changed or the extraction voltage increased.